HaRav Finkel zt"l
On Shabbos, in addition to the general mitzvos of keeping Shabbos as summarized in the commandments Zochor and Shomor, there is a mitzvah of oneg Shabbos. This is learned from the nevi'im, Vekoroso laShabbos oneg - and you shall call the Shabbos oneg — pleasure.
This includes having pleasure on Shabbos in all manner of ways, including having three Shabbos meals that are honored with meat,fish and other delicacies.
Nonetheless, said HaRav Aryeh Finkel zt"l in the talks he gave on leil Shabbos, it does not mean pure pleasure alone, without any purpose or reason. The idea is that since on Shabbos there is revealed the Kingship of the King of Kings, we rejoice in this and express our joy in the pleasures which we experience in honor of the holy Shabbos,
This is what we say in davening: Yismechu bemalchusecha shomrei Shabbos vekor'ei oneg — we are happy with your Kingship that is revealed to us through the Shabbos Queen and therefore we call for oneg — pleasure. We make Shabbos pleasurable in so many ways, that all will be sated and will enjoy Your Goodness.
Only those who call for pleasure on Shabbos will enjoy the pleasures of the Osid lovo, since in order to fully partake of the simchah in the future when malchus Shomayim is revealed in the world it is not enough just to keep the limitations of Shabbos. One must also express the simchah that is involved, Therefore, only those who fulfill the mitzvah of oneg Shabbos with the simchah at the Kingship of the Creator are the ones who will enjoy His Goodness at the time of the ultimate Redemption.
With this we should observe what the characteristics of the meals of Shabbos should be. They should express simchah with Malchus Shomayim, simchah in honor of Hashem Yisborach Who is with us and does us so much good. Sometimes we forget the main point and purpose of the oneg Shabbos and just concentrate on the pleasure itself,
What is asked of us? Only a bit of awareness that we testify to our Creator and that the pleasures of Shabbos kodesh are not the ends in themselves but the means to rejoice in the Kingship of Hashem Yisborach, and to dine with Him and in His honor.
That is the true fulfillment of: Yismechu bemalchusecha shomrei Shabbos vekor'ei oneg!